The "easy" way I did that was to go to my main Library (not User Library) and the command+F (Find) all files of the kind "Music," which should give you a list of all of the .aif and .caf files in all of your GarageBand folders. You can then sort by name and delete all of the old .aif files.
I had a total of 510 aiff files consuming 491 MB of space on my hard drive. They were duplicates between old versions of GarageBand and version 10.0.2. I deleted all of the duplicate .aif versions, and then rebuilt the loops index to get a clean, non-duplicated listing.
While I can't guarantee that I got all of my duplicates, and certainly a different combination of legacy files may exist on your computer, here's a listing of the 510 aiff files that I deleted. I hope this helps you!
70s Ballad Piano 01.aif
70s Ballad Piano 02.aif
80s Dance Bass Synth 06.aif
80s Dance Bass Synth 07.aif
80s Dance Pad 01.aif
80s Pop Synthesizer 01.aif
Acoustic Noodling 01.aif
Acoustic Noodling 02.aif
Acoustic Noodling 03.aif
Acoustic Picking 02.aif
Acoustic Picking 03.aif
Acoustic Picking 04.aif
Acoustic Picking 06.aif
Acoustic Picking 07.aif
Acoustic Picking 08.aif
Acoustic Picking 09.aif
Acoustic Picking 18.aif
Acoustic Slide 02.aif
Acoustic Slide 06.aif
Acoustic Vamp 01.aif
Acoustic Vamp 04.aif
Alternative Rock Bass 01.aif
Alternative Rock Bass 02.aif
Alternative Rock Bass 03.aif
Alternative Rock Bass 04.aif
Asian Gamelan 01.aif
Blip Synth 01.aif
Blue Jazz Organ Riff 01.aif
Blue Jazz Organ Riff 02.aif
Blue Jazz Organ Riff 03.aif
Blue Jazz Organ Riff 04.aif
Blue Jazz Organ Riff 06.aif
Blue Jazz Organ Riff 07.aif
Blue Jazz Piano 01.aif
Blue Jazz Piano 02.aif
Blues Guitar 01.aif
Blues Guitar 02.aif
Blues Guitar 03.aif
Blues Harmonica 02.aif
Blues Harmonica 03.aif
Blues Harmonica 06.aif
Blues Harmonica 08.aif
Blues Harmonica 10.aif
Bongo Groove 01.aif
Bongo Groove 02.aif
Bongo Groove 05.aif
Ceramic Drum 01.aif
Chunky Metal Guitar 01.aif
Classic Rock Guitar 01.aif
Classic Rock Guitar 02.aif
Classic Rock Organ 01.aif
Classic Rock Organ 02.aif
Classic Rock Piano 01.aif
Classic Rock Piano 02.aif
Classic Rock Piano 03.aif
Classic Rock Piano 04.aif
Classic Rock Piano 05.aif
Classic Rock Piano 06.aif
Classic Rock Steel 01.aif
Classic Rock Steel 02.aif
Classic Rock Steel 03.aif
Classic Rock Steel 04.aif
Classic Rock Steel 05.aif
Clave 01.aif
Clave 02.aif
Clean Electric Guitar 01.aif
Club Dance Beat 005.aif
Club Dance Beat 006.aif
Club Dance Beat 007.aif
Club Dance Beat 009.aif
Club Dance Beat 030.aif
Club Dance Beat 038.aif
Club Dance Beat 047.aif
Club Dance Beat 049.aif
Club Dance Beat 050.aif
Club Dance Beat 051.aif
Club Dance Beat 053.aif
Club Dance Beat 054.aif
Club Dance Beat 055.aif
Club Dance Beat 063.aif
Conga Groove 01.aif
Conga Groove 02.aif
Conga Groove 04.aif
Conga Groove 05.aif
Conga Groove 07.aif
Conga Groove 09.aif
Contemplative Synth 01.aif
Contemplative Synth 04.aif
Contemplative Synth 05.aif
Cool Upright Bass 02.aif
Cool Upright Bass 03.aif
Cool Upright Bass 04.aif
Cool Upright Bass 06.aif
Cool Upright Bass 07.aif
Cool Upright Bass 09.aif
Cool Upright Bass 11.aif
Cool Upright Bass 13.aif
Cool Upright Bass 14.aif
Cop Show Clav 02.aif
Cop Show Clav 04.aif
Cop Show Clav 05.aif
Cop Show Clav 06.aif
Country Guitar 01.aif
Deep Electric Piano 01.aif
Deep Pad 01.aif
Delicate Piano 01.aif
Delicate Piano 02.aif
Delicate Piano 03.aif
Delicate Piano 04.aif
Delicate Piano 05.aif
Delicate Piano 11.aif
Dumbek Beat 01.aif
Eastern Santoor 01.aif
Echoing String 01.aif
Echoing String 02.aif
Echoing String 03.aif
Echoing String 04.aif
Edgy Rock Bass 01.aif
Edgy Rock Bass 02.aif
Edgy Rock Bass 03.aif
Edgy Rock Bass 04.aif
Edgy Rock Bass 05.aif
Edgy Rock Bass 06.aif
Edgy Rock Bass 07.aif
Edgy Rock Guitar 01.aif
Edgy Rock Guitar 02.aif
Edgy Rock Guitar 04.aif
Edgy Rock Guitar 05.aif
Edgy Rock Guitar 06.aif
Edgy Rock Guitar 07.aif
Edgy Rock Guitar 08.aif
Edgy Rock Guitar 09.aif
Edgy Rock Guitar 10.aif
Edgy Rock Guitar 11.aif
Edgy Rock Guitar 12.aif
Edgy Rock Guitar 13.aif
Edgy Rock Guitar 14.aif
Edgy Rock Guitar 15.aif
Effected Beat 03.aif
Effected Beat 04.aif
Effected Drum Kit 02.aif
Electric Guitar Swell 01.aif
Electric Slap 01.aif
Electric Slap 02.aif
Electric Slap 03.aif
Electric Slap 08.aif
Electric Slap 12.aif
Electric Slap 16.aif
Electro Beat 01.aif
Electronic Drum Beat 01.aif
Electronic Drum Beat 02.aif
Electronic Drum Beat 03.aif
Emotional Piano 01.aif
Emotional Piano 02.aif
Exotic Beat 01.aif
Exotic Beat 05.aif
Exotic Sarod 01.aif
Far East Drums 02.aif
Folk Mandolin 01.aif
Folk Mandolin 02.aif
Folk Mandolin 03.aif
Funky Electric Guitar 01.aif
Funky Electric Guitar 02.aif
Funky Electric Guitar 03.aif
Funky Electric Guitar 04.aif
Funky Electric Guitar 05.aif
Funky Electric Guitar 06.aif
Funky Electric Guitar 07.aif
Funky Latin Drums 01.aif
Funky Latin Drums 02.aif
Funky Latin Drums 03.aif
Funky Latin Drums 10.aif
Funky Latin Drums 12.aif
Funky Muted Bass 01.aif
Funky Muted Bass 02.aif
Fusion Electric Piano 01.aif
Fusion Electric Piano 02.aif
Fusion Electric Piano 03.aif
Groovy Electric Bass 05.aif
Groovy Electric Bass 06.aif
Groovy Electric Bass 11.aif
Groovy Electric Bass 12.aif
Groovy Electric Bass 14.aif
Groovy Electric Bass 16.aif
Groovy Electric Bass 17.aif
Hip Hop Beat 01.aif
Hip Hop Beat 02.aif
Hip Hop Beat 03.aif
Hip Hop Beat 04.aif
Hip Hop Beat 05.aif
Indian Tabla 02.aif
Island Reggae Bass 01.aif
Island Reggae Bass 02.aif
Island Reggae Bass 03.aif
Island Reggae Organ 01.aif
Island Reggae Organ 02.aif
Island Reggae Organ 03.aif
Latin Lounge Piano 01.aif
Latin Lounge Piano 05.aif
Latin Lounge Piano 08.aif
Latin Lounge Piano 11.aif
Live Edgy Drums 01.aif
Live Edgy Drums 03.aif
Live Edgy Drums 04.aif
Live Edgy Drums 05.aif
Live Edgy Drums 07.aif
Live Edgy Drums 10.aif
Live Edgy Drums 11.aif
Live Edgy Drums 12.aif
Live Edgy Drums 13.aif
Live Edgy Drums 14.aif
Long Crash Cymbal 01.aif
Long Crash Cymbal 02.aif
Lounge Vibes 01.aif
Lounge Vibes 02.aif
Lounge Vibes 03.aif
Lounge Vibes 04.aif
Lounge Vibes 05.aif
Lounge Vibes 06.aif
Middle Eastern Oud 01.aif
Mini Mono Synth 01.aif
Mini Mono Synth 02.aif
Modern Rock Drums 01.aif
Modern Rock Drums 02.aif
Modern Rock Drums 03.aif
Modern Rock Drums 04.aif
Modern Rock Drums 05.aif
Modern Rock Drums 06.aif
Modern Rock Drums 07.aif
Modern Rock Drums 08.aif
Modern Rock Drums 09.aif
Modern Rock Drums 10.aif
Modern Rock Drums 11.aif
Modern Rock Drums 12.aif
Modern Rock Guitar 01.aif
Modern Rock Guitar 02.aif
Modern Rock Guitar 03.aif
Modern Rock Guitar 04.aif
Modern Rock Guitar 05.aif
Modern Rock Guitar 06.aif
Modern Rock Guitar 07.aif
Modern Rock Guitar 08.aif
Modern Rock Guitar 09.aif
Modern Rock Guitar 10.aif
Modern Rock Guitar 11.aif
Modern Rock Guitar 12.aif
Modern Rock Guitar 13.aif
Modern Rock Guitar 14.aif
Modern Rock Guitar 15.aif
Modern Rock Guitar 16.aif
Modern Rock Guitar 17.aif
Modern Rock Guitar 18.aif
Modern Rock Guitar 19.aif
Modern Rock Guitar 20.aif
Modern Rock Guitar 21.aif
Modern Rock Guitar 22.aif
Modern Rock Guitar 23.aif
Modern Rock Guitar 24.aif
Modern Rock Guitar 25.aif
Modern Rock Guitar 26.aif
Moody Electric Piano 01.aif
Moody Electric Piano 02.aif
Moody Electric Piano 03.aif
Moody Electric Piano 05.aif
Motown Drummer 01.aif
Motown Drummer 02.aif
Motown Drummer 03.aif
Motown Drummer 04.aif
Motown Drummer 05.aif
Motown Drummer 06.aif
Motown Drummer 07.aif
Motown Drummer 08.aif
Motown Drummer 09.aif
Motown Drummer 10.aif
Motown Drummer 11.aif
Motown Drummer 12.aif
Motown Drummer 13.aif
Motown Drummer 14.aif
Motown Drummer 15.aif
Motown Drummer 16.aif
Motown Drummer 17.aif
Motown Drummer 18.aif
Motown Drummer 19.aif
Motown Drummer 20.aif
Muted Rock Bass 01.aif
Muted Rock Bass 02.aif
Muted Rock Bass 03.aif
Muted Rock Bass 04.aif
Muted Rock Bass 05.aif
Natural Drum Kit 01.aif
Natural Drum Kit 02.aif
Natural Drum Kit 03.aif
Natural Drum Kit 04.aif
Natural Drum Kit 05.aif
Natural Drum Kit 06.aif
Natural Drum Kit 07.aif
Natural Drum Kit 08.aif
Natural Drum Kit 09.aif
Natural Drum Kit 10.aif
Natural Drum Kit 11.aif
Natural Drum Kit 12.aif
Natural Drum Kit 13.aif
Natural Drum Kit 14.aif
Old School Funk Line 03.aif
Old School Funk Line 05.aif
Orchestra Strings 01.aif
Orchestra Strings 02.aif
Orchestra Strings 03.aif
Orchestra Strings 04.aif
Orchestra Strings 05.aif
Orchestra Strings 06.aif
Orchestra Strings 07.aif
Orchestra Strings 08.aif
Orchestra Strings 09.aif
Orchestra Strings 10.aif
Orchestra Strings 11.aif
Orchestra Strings 12.aif
Orchestra Strings 13.aif
Percussion Combo 02.aif
Percussion Combo 04.aif
Percussion Combo 06.aif
Percussion Combo 08.aif
Percussion Combo 09.aif
Picked Steel String 01.aif
Picked Steel String 02.aif
Picked Steel String 03.aif
Picked Steel String 04.aif
Picked Steel String 05.aif
Picked Steel String 06.aif
Picked Steel String 07.aif
Picked Steel String 08.aif
Picked Steel String 09.aif
Picked Steel String 10.aif
Picked Steel String 11.aif
Picked Steel String 12.aif
Picked Steel String 13.aif
Picked Steel String 14.aif
Picked Steel String 15.aif
Picked Steel String 16.aif
Picked Steel String 17.aif
Picked Steel String 18.aif
Picked Steel String 19.aif
Picked Steel String 20.aif
Pluck Synth 01.aif
Pluck Synth 02.aif
Pluck Synth 03.aif
Plucky Guitar Loop 01.aif
Plucky Guitar Loop 02.aif
Pop Piano 01.aif
Pop Piano 05.aif
Pop Piano 06.aif
Pop Rock Guitar 01.aif
Pop Rock Guitar 02.aif
Reggae Toy Piano 01.aif
Reggae Toy Piano 02.aif
RnB Bass 01.aif
RnB Bass 02.aif
RnB Horn Section 01.aif
RnB Horn Section 02.aif
RnB Horn Section 04.aif
RnB Horn Section 05.aif
RnB Horn Section 12.aif
Rock Bass 01.aif
Rock Bass 02.aif
Rock Bass 03.aif
Rock Bass 04.aif
Rock Bass 05.aif
Rock Bass 06.aif
Rock Bass 07.aif
Rock Bass 08.aif
Rock Bass 09.aif
Rock Bass 10.aif
Rock Bass 11.aif
Rock Bass 12.aif
Rock Bass 13.aif
Rock Bass 14.aif
Rock Bass 15.aif
Rock Bass 16.aif
Rock Bass 17.aif
Rock Bass 18.aif
Rock Bass 19.aif
Rock Guitar Riff 01.aif
Rock Guitar Riff 02.aif
Rock Guitar Riff 03.aif
Round Funk Bass 01.aif
Round Funk Bass 02.aif
Round Funk Bass 03.aif
Round Funk Bass 04.aif
Round Latin Bass 02.aif
Round Latin Bass 04.aif
Round Latin Bass 05.aif
Round Latin Bass 06.aif
Round Latin Bass 07.aif
Rural Banjo 01.aif
Rural Banjo 02.aif
Shaker 01.aif
Shaker 02.aif
Shaker 05.aif
Shaker 07.aif
Shaker 10.aif
Shaker 11.aif
Shaker 14.aif
Shaker 17.aif
Short Crash Cymbal 01.aif
Short Crash Cymbal 02.aif
Southern Bass 01.aif
Southern Bass 02.aif
Southern Bass 03.aif
Southern Bass 04.aif
Southern Bass 05.aif
Southern Rock Drums 01.aif
Southern Rock Drums 02.aif
Southern Rock Drums 03.aif
Southern Rock Drums 04.aif
Southern Rock Drums 05.aif
Southern Rock Drums 06.aif
Southern Rock Guitar 01.aif
Southern Rock Guitar 02.aif
Southern Rock Guitar 03.aif
Southern Rock Guitar 04.aif
Southern Rock Guitar 05.aif
Southern Rock Organ 01.aif
Southern Rock Organ 02.aif
Southern Rock Organ 03.aif
Southern Rock Organ 04.aif
Southern Rock Organ 05.aif
Southern Rock Organ 06.aif
Southern Rock Organ 07.aif
Southern Rock Piano 01.aif
Southern Rock Piano 02.aif
Southern Rock Piano 03.aif
Southern Rock Piano 04.aif
Southern Rock Piano 05.aif
Spacey Electric Piano 01.aif
Spacey Electric Piano 02.aif
Spacey Electric Piano 03.aif
Spacey Electric Piano 04.aif
Spacious Guitar 01.aif
Spacious Guitar 02.aif
Spacious Guitar 04.aif
Steel String Slide 01.aif
Steel String Slide 02.aif
Steel String Slide 03.aif
Steel String Slide 04.aif
Steel String Slide 05.aif
Steel String Slide 06.aif
Steel String Slide 07.aif
Strummed Acoustic 01.aif
Strummed Acoustic 02.aif
Strummed Acoustic 03.aif
Strummed Acoustic 04.aif
Synth Array 01.aif
Synth Array 02.aif
Synth Array 07.aif
Synth Array 09.aif
Synth Array 10.aif
Synth Array 11.aif
Synth Array 13.aif
Synth Array 18.aif
Synth Array 19.aif
Synth Array 20.aif
Synth Tone Bass 01.aif
Synth Tone Bass 02.aif
Synthesizer Bass 02.aif
Synthesizer Bass 03.aif
Synthesizer Bass 06.aif
Synthesizer Bass 07.aif
Synthesizer Bass 11.aif
Tambourine 01.aif
Tambourine 02.aif
Tambourine 04.aif
Tambourine 07.aif
Techno Bass 01.aif
Techno Bass 04.aif
Techno Bass 05.aif
Techno Synth 02.aif
Techno Synth 05.aif
Techno Synth 06.aif
Texas Blues Guitar 01.aif
Thick Guitar Delay 01.aif
Thumpy Hip Hop Bass 01.aif
Thumpy Hip Hop Bass 02.aif
Thumpy Hip Hop Bass 03.aif
Thumpy Hip Hop Bass 04.aif
Thumpy Hip Hop Bass 05.aif
Upbeat Electric Piano 01.aif
Upbeat Electric Piano 02.aif
Upbeat Electric Piano 03.aif
Upright Funk Bass 01.aif
Upright Funk Bass 02.aif
Upright Funk Bass 04.aif
Upright Funk Bass 05.aif
Upright Funk Bass 06.aif
Upright Funk Bass 08.aif
Upright Funk Bass 09.aif
Upright Funk Bass 12.aif
Upright Funk Bass 14.aif
Upright Funk Bass 15.aif
Vintage Funk Kit 01.aif
Vintage Funk Kit 02.aif
Vintage Funk Kit 03.aif
Vintage Funk Kit 04.aif
Vintage Funk Kit 05.aif
Walking Upright Bass 01.aif
Walking Upright Bass 02.aif
Walking Upright Bass 06.aif
Walking Upright Bass 08.aif
Woody Latin Bass 01.aif
Woody Latin Bass 02.aif
World Drums 01.aif